This challenge is part of the Fortnite Chapter 2, Season 1 "The Lowdown" Mission. You'll have to heal a total of 200 HPs on your teammates in any mode.
We particularly recommend Team Rumble to validate this challenge easily. You'll have about a dozen teammates to heal, and loots are still as numerous as ever. As a bonus, you can reappear at will to heal your allies on the line.
Keep in mind that the Bandage Bazooka takes two slots in the inventory, but recharges automatically over time. You don't have to change weapons if you run out of bandages! Keep an eye out for supply drops: they often contain Bazookas.